password auth: Write a real type for navigation prop.

This mirrors the assumptions on this type that are provided in
navActions when we navigate to this screen.

src/start/PasswordAuthScreen.js

@@ -1,8 +1,8 @@
-/* @flow */
+/* @flow strict-local */
 import { connect } from 'react-redux';
-
 import React, { PureComponent } from 'react';
 import { View, StyleSheet } from 'react-native';
+import type { NavigationScreenProp } from 'react-navigation';
 
 import type { Auth, Dispatch, GlobalState } from '../types';
 import { fetchApiKey } from '../api';
@@ -28,7 +28,7 @@ const styles = StyleSheet.create({
 type Props = {|
   partialAuth: Auth,
   dispatch: Dispatch,
-  navigation: Object,
+  navigation: NavigationScreenProp<{ params: {| requireEmailFormat: boolean |} }>,
 |};
 
 type State = {|